68 lines
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68 lines
8.3 KiB
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Chapter 6. Error handling</title><link rel="stylesheet" type="text/css" href="manual.css"><meta name="generator" content="DocBook XSL Stylesheets V1.76.0"><link rel="home" href="index.html" title="JpGraph Manual"><link rel="up" href="pt02.html" title="Part II. Basic graph creation"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 6. Error handling</th></tr><tr><td width="20%" align="left"> </td><th width="60%" align="center">Part II. Basic graph creation</th><td width="20%" align="right"> </td></tr></table><hr></div><div class="chapter" title="Chapter 6. Error handling"><div class="titlepage"><div><div><h2 class="title"><a name="chap.error-handling"></a>Chapter 6. Error handling</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="ch06.html#id2495302">6.1. The problem with error messages and images</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch06.html#id2495400">6.1.1. Catching errors in other parts of the system while creating images</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch06s02.html">6.2. Available error messages</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch06s02.html#sec2.localizing-errmsg">6.2.1. Localizing error messages</a></span></dt><dt><span class="sect2"><a href="ch06s02.html#id2495515">6.2.2. Error messages on a production server</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch06s03.html">6.3. Using PHP Exceptions</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch06s03.html#id2495896">6.3.1. Selecting between text and image based error handling</a></span></dt><dt><span class="sect2"><a href="ch06s03.html#id2495901">6.3.2. Writing error message to a log file (or system logger)</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch06s04.html">6.4. Adding a new locale</a></span></dt></dl></div>
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<div class="sect1" title="The problem with error messages and images"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2495302"></a>The problem with error messages and images</h2></div></div></div>
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<p>Why a whole chapter on error handling? The answer is that error handling with
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scripts that produces images requires a slightly different approach than normal
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error messages. In order to appreciate the added complexity one needs to consider
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how a graph script is called. Usually a graph script is called from an
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<span class="markup"><img></span> tag. This means that the client (e.g. the browser)
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expect image data to be sent from the script. If we instead send a textual error
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message the browser will not be able to display anything. It was expecting a stream
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with image data and the data received was a representation of an error message which
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of course cannot be interpreted as an image.</p>
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<p>To handle this problem the error messages produced by the library are normally
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generated as an image. A typical error message can be seen in <a class="xref" href="ch06.html#fig.errmsg-ex1" title="Figure 6.1. Typical image error message">Figure 6.1. Typical image error message</a></p>
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<div class="figure"><a name="fig.errmsg-ex1"></a><p class="title"><b>Figure 6.1. Typical image error message</b></p><div class="figure-contents">
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<div class="mediaobject"><img src="images/errmsg-ex1.png" alt="Typical image error message"></div>
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</div></div><br class="figure-break">
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<p>There is one exception to this rule and that is in the case of the "Headed already
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sent" error message. This means that the client has already received an header and
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with probability bordering on certainty this is a text header. Hence in this case it
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does not make sense to send an image so this, and only this, error message is sent
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as a normal "text/html" type message. A typical example how this message is shown in
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the browser can be seen in <a class="xref" href="ch06.html#fig.errmsg-header-sent" title='Figure 6.2. The "Header already sent error message"'>Figure 6.2. The "Header already sent error message"</a></p>
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<div class="figure"><a name="fig.errmsg-header-sent"></a><p class="title"><b>Figure 6.2. The "Header already sent error message"</b></p><div class="figure-contents">
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<div class="mediaobject"><img src="images/errmsg-headersent.png" alt='The "Header already sent error message"'></div>
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</div></div><br class="figure-break">
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<p>When you get this error (and we mean when - not if) take a very close look at the
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text in red. This will tell you in what file and what line the erroneous output
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started. In most cases this will be an innocent looking space or a tab character. It
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could also be caused by multiple newlines at the end of a file. Since by definition
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these mistakes can be hard to spot since spaces and tabs are not normally visible in
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an editor so take care! </p>
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<div class="sect2" title="Catching errors in other parts of the system while creating images"><div class="titlepage"><div><div><h3 class="title"><a name="id2495400"></a>Catching errors in other parts of the system while creating images</h3></div></div></div>
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<p>The problem described in the previous section will also apply to error
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messages that are generated directly from PHP as well. In order to be able to
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see any potential error messages during development it is possible to instruct
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the library to intercept PHP error messages and convert them to image messages
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instead. This is controlled by the two defines
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<code class="code">INSTALL_PHP_ERR_HANDLER</code> and <code class="code">CATCH_PHPERR</code> define in
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<code class="filename">jpg-config.inc.php</code>
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</p>
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<p>
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</p><div class="variablelist"><dl><dt><span class="term"><code class="code">INSTALL_PHP_ERR_HANDLER</code></span></dt><dd>
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<p>Setting this define to true will make the library install a
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custom error handler which will catch all PHP error messages and
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convert them into an image like what can be seen in <a class="xref" href="ch06.html#fig.errmsg-ex1" title="Figure 6.1. Typical image error message">Figure 6.1. Typical image error message</a>. </p>
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<p>By default this is disabled.</p>
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</dd><dt><span class="term"><code class="code">CATCH_PHPERR</code></span></dt><dd>
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<p>This defines control whether the library shall check for any
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generated error message that are stored in the global PHP
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pre-defined variable <code class="code">php_errmsg</code>. This can be very
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useful during development if an error has occurred prior to
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calling the graph script. This error will then be displayed as
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an image.</p>
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<p>By default this is enabled.</p>
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</dd></dl></div><p>
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</p>
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</div>
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</div>
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